The present invention concerns two-phase or three-phase AC motors and, more particularly, a device to control the speed of such motors.
The invention shall be described in its application to the field of medical radiology where it is necessary to control the rotational speed of the anode of an X-ray emitting tube. As shown schematically in FIG. 1, a tube such as this is generally formed like a diode, that is, with a cathode 11 and an anode 12 or anti-cathode, these two electrodes 11 and 12 being enclosed in a vacuum-tight casing 13 when enables the electrical insulation to be achieved between these two electrodes. The cathode 11 produces beam of electrons, and the anode 12 receives these electrons on a small area which is the focal spot from where the X-ray beams are emitted.
When the high supply voltage is applied to the terminals of the cathode 11 and the anode 12, so that the cathode is at the negative potential, a current called an anode current is set up in the circuit, through a generator 14 producing the high supply voltage. The anode current crosses the space between the cathode and the anode in the form of a beam of electrons which bombard the focal spot.
A small proportion (1%) of the energy spent to produce the electron beam is converted into X-rays. Hence, given also the high values of instantaneous power (of the order of 1 to 100 KW) and the small dimensions of the focal spot (of the order of one millimeter), manufacturers have long been making rotating anode X-ray tubes where the anode is put into rotation to distribute the thermal flux over a ring called a focal ring with a greater area than that of the focal spot, the usefulness thereof being all the greater as the rotation speed is high, generally between 3,000 and 12,000 rpm.
The rotating anode 12, which is of a standard type, has the general shape of a disk with an axis of symmetry 17 around which it is put into rotation by means of an electrical motor. The electrical motor has a stator 15 located outside the casing 13 and a rotor 16 mounted in the casing and positioned along the axis of symmetry 17, the rotor being mechanically fixed to the anode by means of a supporting shaft 18. This motor is generally of the asynchronous type so that it does necessitate the creation of an inductive field by the rotor. The energy dissipated in a tube such as this is high, and it is therefore designed to be cooled. To this end, the tube is enclosed in a chamber or sheath 19 wherein a cooling liquid such as oil, is made to flow.
The high-speed rotation of the anode results in fast wearing out of the bearings of the motor. Hence, to prolong their lifetime as well as to reduce the heat losses of the motor which are dissipated in the sheath enclosing the X-ray tube, the anode is not permanently driven at high speed. This means that there is provision for at least two speeds of rotation, a high speed for the radiological exposure time and a lower speed between two exposures, the latter speed being possibly zero.
Besides, in the prior art, the same X-ray tube can be used to create two different X-ray sources which correspond to different focal spots by their size and to different flow rates. This results in different operating conditions, and it is common to have a matching rotational speed for each type of focal spot. Thus, for a focal spot of 0.3 mm, the rotational speed will be 3000 rpm while it will be 9000 rpm for a focal spot of 0.1 mm wherein the energy is concentrated on a smaller area.
The graphs of FIG. 2 show, by way of example, two operating cycles of the rotating anode of an X-ray tube, one graph 20 for a 0.3 mm focal point and another graph 21 for a 0.1 mm. focal point. The two cycles are identical and comprise a first stage A which corresponds to the starting up of the motor, a second stage B for maintaining the speed (3,000 rpm or 9,000 rpm) and a third braking stage C which lasts until the motor is stopped.
The motors that are used to make the rotating anodes are generally of the two-phase type and the electrical diagram that enables an operating cycle to be done is, for example, that of FIG. 3. In this FIG. 3, the motor 30 is shown in the form of a winding called a main phase winding 31 and a winding called auxiliary winding 32 in series with a phase shift capacitor 33. For the frequency considered, this capacitor 33 achieves the supply in quadrature of both windings 31 and 32. These two windings 31 and 32 are supplied with a single-phase A.C. voltage 34 through a transformer 35 and relay contacts 36 and 37 series mounted on the supply conductors 38 and 39. The common point of the windings 31 and 32 is directly connected to the secondary winding of the transformer 35. Moreover, the two conductors 38 and 39 are connected by a conductor 29, positioned between the relay contacts 36 and 37.
When the relays 36 and 37 are actuated, the two windings 31 and 32 are supplied with the normal voltage by the conductor 38, and the motor 30 starts up (stage A). When the relay 36 is then released, the windings 31 and 32 are supplied with the reduced voltage by the conductor 39: this is the stage B.
To obtain the braking of the motor, it is planned to open the contacts of the relay 37 and to inject a D.C. current into the main winding 31, for example. To this effect, the two terminals of the winding 31 are connected to a rectifier circuit 40 by means of the contacts of a relay 41. Thus, when the relays 36 and 37 are deactivated while the relay 41 is actuated, a current flows in the main winding and brakes the motor 30.
With a device such as this for supplying the motor 30, this motor runs at a speed of 3,000 rpm when the supply frequency is 50 Hertz. To obtain a rotational speed of 9,000 rpm, it is enough to triple the frequency of the mains supply by using, for example, a saturated air-gap transformer and by changing the phase shift capacitor by means of a change-over switch (not shown).
To obtain rotational speeds of the motor that are different from those imposed by the mains (3,000 rpm or 9,000 rpm), it is necessary to use a converter. The use of a converter is also necessary when the supply is a D.C. supply, for example for battery-operated movable radiological instruments.
One of the ways adopted is to use a single-phase converter which supplies a two-phase motor, the auxiliary phase of which is in series with a phase shift capacitor. This approach has the drawback of requiring switch-over operations, so as to match the phase shift capacitors with the speed and to obtain braking. Furthermore, there is no optimization of the converter-motor unit for, particularly, on the one hard the capacitor achieves the desired phase shift with a low precision, depending on its own tolerance and on the tolerance of the motor and, on the other hand, it causes an increase in the current harmonics components in the auxiliary phase.
To overcome the drawbacks of this first approach two single-phase converters in quadrature and one two-phase motor without phase shift capacitor are used. The electrical circuit diagram is that of FIG. 4. The winding 31 of the main phase is supplied by a first converter 44 represented by four switches 45, 46, 47 and 48, while the winding 32 of the auxiliary phase is supplied by a second converter 49 represented by four switches 50, 51, 52 and 53. For a better understanding, each switch may be considered to consist a transistor or a thyristor associated with an antiparallel diode. A capacitor 54 makes the input filter of the converters 44 and 49 which are supplied with D.C. current by a source 43.
This second approach is a high-cost approach for it uses two converters. Hence, a third approach consists in the use of a motor 66, the stator of which enables a three-phase winding, this winding being supplied by a three-phase converter according to the diagram of FIG. 5. In this figure, the converter 55 has three pairs or couples of switches 56 and 57, 58 and 59, 60 and 61, for which each common point A, B or C is connected to a winding 62 for the switches 56 and 57, to a winding 63 for the switches 58 and 59 and to a winding 64 for the switches 60 and 61. In this schematic diagram, the filtering capacitor is referenced 65. The opening and closing of the switches 56 to 60 are controlled by a device 67 which gives control signals of said switches. If the control signals are considered to be such that the waveforms VA, VB, VC, measured between the common points A, B, C of the switches and the negative supply pole are given by the graphs of the FIGS. 6-a, 6-t, 6-c phase shifted by 120.degree. with respect to one another. The graphs of FIGS. 6-d, 6-e and 6-f give the result of the combination of these waveforms with one another such that the FIG. 6-d corresponds to VA-VB, FIG. 6-e corresponds to VB-VC and 6-f corresponds to VC-VA. These waveforms, commonly called pseudosinusoidal waves, are phase shifted by 120 degrees with respect to one another.
In this third approach, the easiest embodiment of the three-phase coil of the motor enables an improvement in the performance characteristics of the motor, thus enabling a shorter speed build-up time (stage A). Furthermore, the operation of such a device leads to the concellation, from the motor, of the current harmonics components belonging to an order which is a multiple of three. These harmonics components, like the intermediate harmonics components, do not give any useful torque but, on the contrary, create stray currents and give rise to losses. Finally, a lightening of the input filter is obtained for the frequency of the ripple imposed by the three-phase motor is tripled. This reduces the value of the capacitance of the filtering capacitor 65.
However, such an approach can be implemented only if the stator has a number of notches which is a multiple of three so as to enable a three-phase winding.
Furthermore, in view of the use of two-phase motors to date, the compatibility of the speed control device with this type of motor must be preserved.